The inner ear (or cochlea) is a small double channel helical-shaped feature in the skull containing a key part of the human hearing mechanism. Nerves are arranged in the center of the cochlea (modiolus) and branch out into the basilar membrane. This membrane separates the scala tympani channel on one side, and the combined scala vestibuli and scala media channels on the other side. Since the scala media adjoins the scala vestibuli via a very thin membrane, for convenience, both of these scalae are referred to herein as the scala vestibuli. Sound waves are collected by the external ear and transmitted to the tympanic membrane (or ear drum), which then transfers the vibrations to the fluid in the scala vestibuli via the three auditory ossicles in the middle ear. The acoustic waves in the scala vestibuli are further transmitted to the scala tympani via a small opening (the helicotrema) at the apical end of the cochlea. A resonance is set up in the two scalae whereby the acoustic waves in the scalae are out of phase with each other, thereby causing the thin basilar membrane separating the two scalae to oscillate. A complex arrangement of hair cells on the basilar membrane are activated by the movement of said membrane, which trigger a nerve response which is interpreted by the brain as speech.
One cause of profound deafness is the loss or destruction of hair cells within the cochlea. Without these hair cells, there is no tactile input to the auditory nerves. Although there is currently no method for replacing the hair cells, it has been demonstrated that the auditory nerves respond directly to electrical stimulation. This direct electrical stimulation of the auditory nerve in the cochlea is the basis of modern cochlear implants. A comprehensive introduction of the history of the development of cochlear implants is given by, for example, G. M. Clark, et. al., Chapter 1, pages 1-14, in Cochlear Prostheses, edited by G. M. Clark, Y. C. Tong and J. F. Patrick, distributed in the U.S.A. by Churchill Livingstone Inc., New York, N.Y. (ISBN 0-443-03582-2).
The current state of the art of electrode devices used in commercial cochlear implants consists of one or more electrodes on a single probe inserted into the scala tympani. The electrodes are electrically connected to an electronic package anchored in the mastoid bone behind the ear. An acoustic signal is received by a microphone located on an external body-mounted electronics package and, using a speech processing strategy, is converted for transmission, through the skin, to the unit anchored in the mastoid bone.
The electrodes are generally comprised of small platinum/iridium balls or circular platinum rings connected internally by thin wires, with the electrodes and wires held together by an inert silicone carrier. Various interconnection combinations between the electrodes on the single implant probe are used to create a map or stimulation pattern within the cochlea. To date, data from implanted patients indicate highly variable speech recognition results from patient to patient. The reason for this high variability is not known. In general, there is some suggestion that more electrodes can provide improved patient hearing percepts. Although implants with only one electrode have been successful, there is growing evidence that multi-electrode implants, especially combined with advanced speech processing technology, can provide superior speech understanding for the patient. However, there is no firm evidence as to the optimum number of electrodes, although there is some support for the notion that more electrodes is better. This technology is described in, for example, T. J. Balkany, editor of The Otolaryngologic Clinics of North America, Vol. 19, No. 2, May, 1986, titled The Cochlear Implant, (ISSN 0030-6665), and, more recently, in the Abstract Proceedings from the Conference on Implantable Auditory Prostheses, Aug. 17-21, 1997, held in Pacific Grove, Calif. U.S.A.
Many attempts have been made to design electrodes that can be positioned near the lateral walls of the scala tympani, especially near the modiolus. For example, Kuzma describes in U.S. Pat. Nos. 5,545,219 and 5,645,585 means for positioning a flexible rod-like carrier held by a positioning member to position the electrodes near the modiolus. In U.S. Pat. No. 5,578,084, Kuzma further describes a method for altering the shape of an electrode in situ through the use of bio-absorbing materials so as to position it closer to the medial wall of the cochlea. Parker et. al. details the use of bio-resorbable materials to allow an electrode to change shape after insertion in the cochlea in U.S. Pat. No. 5,653,742.
All cochlear electrode arrays to date have been developed for insertion into only the scala tympani. However, in some cases it is not possible to insert the electrodes into the scala tympani due to ossification. One option for the surgeon is to then attempt to implant the electrodes into the scala vestibuli. Evidence to date suggests that implantation of electrodes into the scala vestibuli results in similar hearing percepts by the patient compared to implants in the scala tympani (A. J. Gulya, et. al., Arch. Otolaryngo Head Neck Surg./Vol. 122, February 1996). Thus, it appears that from a medical perspective, implantation of an electrode array into either scala is viable.
No data are available on the implantation and use of electrode arrays in both scalae of the cochlea although it is interesting to note that Hansen in U.S. Pat. No. 4,261,372 describes the use of a two part electrode array where one part of the electrode array is inserted into the first turn of the scala tympani and the other part is inserted into the second turn of the scala tympani. This prior art is a single electrode array comprised of two parts, and configured to enter the same scala to obtain improved insertion depth of the electrode array.
Conventional single probe implants tend to be positioned near the lateral wall in the scala tympani, whereas the stimulatable spiral ganglion cells are located near the modiolus, resulting in high threshold currents, a lack of electric field discrimination and crosstalk effects. The nerve cells in such a configuration are simply too far away from the electrodes to be discretely stimulated.
In addition to nerve proximity, issues have arisen wherein the insertion of the device causes the array to twist, resulting in a non-optimal placement. It is believed that much of the variation in the performance of these electrode arrays results from the imperfect science of device insertion during surgery. Designs which preferentially turn in one plane while maintaining their rigidity in the other plane have been described in, for example, U.S. Pat. No. 5,123,422 and U.S. Pat. No. 4,832,051.
It is noted that the electrode array devices that have been disclosed suffer from a variety of limitations, which predispose them to meeting some criteria while not meeting others. The electrode array criteria that are generally believed to provide for optimum performance are: (1) electrode proximity to functional ganglion cells near the modiolus (i.e. the central core of the cochlea), (2) surgical insertability of the entire electrode array into the scala and (3) density of electrodes (number of electrodes per unit length).
The object of this invention is to provide an implant device that overcomes the limitations of the prior art, as well as a method of surgical implantation of such a device.
It is a further object of this invention to provide an implant which increases selectivity of stimulation sites, localizes the electric field distribution, minimizes the electrical stimulation current required, improves the ease of surgical implantation and positioning, and improves patient speech percepts.